Investigations are directed at understanding (1) the mechanism of chloroquine resistance in Plasmodium falciparum malaria; (2) the expression and variation of the var gene family that modulates the adhesive and antigenic character of parasitized red blood cells; (3) a gene that is genetically linked to invasion proficiency by which malaria parasites enter red blood cells; and (4) a genetic defect of chromosome 12 that adversely affects the development of male gametocytes. The determinant of chloroquine resistance has been localized to a 36 kb segment of P. falciparum chromosome 7 in a genetic cross. DNA sequencing of this segment has identified five transcribed genes, which are being investigated in detail, to determine the mechanism of resistance. Factors responsible for the spread of drug resistance are being examined in epidemiological studies in Mali. Transfection and stable transformation of P. falciparum erythrocytic stages have been developed and are being applied to investigations of candidate chloroquine resistance genes. New selectable markers and methods to improve the efficiency and utility of the transformation are under investigation. A highly diverse family of genes (var) has been characterized that mediates antigenic variation and cytoadherence of parasitized red blood cells. Possible gene conversion events and DNA rearrangements involving telomeres have been identified as potential mechanisms that generate diversity. An unusual pool of sterile RNA transcripts (2 kb) may be involved in var gene rearrangements or expression. A novel gene, ebl-1, has been identified that is linked to invasion of red blood cells by P. falciparum. The 8 kb open reading frame encodes a molecule that is related to known ligands for sialic-acid residues at the erythrocyte surface. A defect that differentially affects the development of male gametocytes has been traced to a spontaneous mutation in a cultivated P. falciparum line. The mutation has been localized to an 800 kb segment of chromosome 12.